As can be imagined, it is vitally important to balance the rotor assembly of a jet engine so as to avoid unstable or erratic rotation. As can also be imagined, the higher the rpm involved, the more critical and accurate the balance must be.
The current level of accuracy available for jet engine blades is 0.05% of each individual blades' moment about the central axis of the jet engine (referred to as "mass-moment"). While this degree of accuracy is highly desirable, it is extremely laborious and time consuming to determine this value for each individual blade for each jet engine being manufactured or refurbished.
One current and highly sophisticated method of measuring the mass-moment of the individual blades is to attach a standard or ideal blade to a balance beam and then to record or balance out the value of this reference blade. Then, as each subsequent blade to be measured is attached to the balance beam, their variance with the reference blade is noted. This will identify any blade that falls outside the allowable range. Unfortunately, these individual blades are not subject to reworking, the purpose of determining their mass-moment is to selectively assemble a balanced rotor for the jet engine.
According to the above method, a highly accurate scale will provide an equally highly accurate value. However, in reality, the location of the attachment of the subsequent blades to the balance beam is critical because any blade not exactly and identically attached as the reference blade will result in an inaccurate readout. An error of only a few thousandths of an inch could produce such a false value because of the accuracy of the scale. Additionally, since a blades' mass-moment is defined as its mass multiplied by the distance of its center of gravity to the measuring location, any such error in location would be magnified accordingly. Consequently, when such a high degree of accuracy is desired, more than accurate equipment is needed, accurate and consistent attachment of the blade to the balance beam is also equally important.
A balance beam (or moment weighing) system now in use requires an operator to manually attach a fan blade to an end of the beam. The potential for error using this method has been discussed above. This system also incorporates a beam dampening device that quickly eliminates any fluctuations of the beam. While an admirable addition, it also introduces error by restricting the free movement of the balance beam. It would be preferable to incorporate the dampening feature into the measuring device itselt (i.e. the scale) rather than the thing being measured (i.e. the movement of the balance beam).
Another error-prone aspect of the above system is the location of the measuring cell. Instead of being located at the fulcrum of the balance beam, it is offset some distance from this location. Since the fulcrum is the point from which mass-moment is determined (it being one end of the distance multiplicand), this system requires the additional conversion of the measured data so as to take into account this offset location. Such an additional step can be a source of unintentional error if the conversion factor is wrong or was improperly calculated. Another source of error that should be eliminated so as to obtain an accurate reading concerns environmental factors that affect the sensitive weighing apparatus. These factors include air currents and temperature gradients that are to be avoided if utmost accuracy is desired.
It is thus an object of this invention to provide an accurate and consistent method of calculating the mass-moment of jet engine blades. Another object of this invention is to provide a system that positions the blades to be weighed in the exact, same, and identical position each time. Still another object of this invention is to provide a means for weighing each blade that will result in an accurate mass-moment value with no error due to measuring cell location or dampening device. A further object of this invention is to provide a system of determining the mass-moment of an individual blade which eliminates or avoids any error that may be due to air currents, temperature gradients or other environmental factors. These and other objects and features of this invention will become evident upon further inspection.